1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to new and improved terminal slowdown control for elevator systems.
2. Description of the Prior Art
It is necessary in elevator systems to provide independent means for detecting an overspeed condition of an elevator car as it approaches a terminal floor. In addition to the speed monitoring function, once an overspeed condition is detected, means must be provided for safely bringing the elevator car to a stop.
U.S. Pat. No. 3,779,346 which is assigned to the same assignee as the present application, discloses terminal slowdown control which includes spaced markers, such as a notched blade, and a detector. The markers and detector are mounted such that there is relative movement between them as the elevator car approaches a terminal floor. The notched blade is preferably mounted in the hatchway, and the detector is mounted on the elevator car such that it is aligned with the blade and will detect the spaced teeth on the blade as it passes them.
The markers or teeth on the blade are non-uniformly spaced, with the spacing being selected to provide the desired speed profile for bringing the elevator car to a stop at the terminal floor. The teeth are spaced successively closer together in the direction of the terminal such that the time required for the car to pass between any adjacent pair will be constant if the deceleration rate of the car is constant. If the time elapsed between adjacent pairs is shorter than a predetermined time, monitoring means, including means for converting the elapsed time to a speed error signal, will detect the overspeed condition and initiate slowdown, i.e., the switching from the normal speed pattern to an auxiliary speed pattern.
The spaced markers, and detector, along with means for converting the elapsed time between the spaced markers to a speed error signal, used for the monitoring function, are also used to generate the auxiliary speed pattern. The speed error signal is summed with a signal responsive to the speed of the elevator car, such as a signal from a tachometer responsive to the elevator drive motor, to provide the required slowdown speed pattern profile.
The notched or toothed blade in U.S. Pat. No. 3,779,346 provides a terminal slowdown signal responsive to car speed, and car speed error, and it is an exponential function of the distance of the elevator car to the terminal floor. While this arrangement provides an excellent terminal slowdown arrangement, the actual car speed must exceed the magnitude of the terminal slowdown speed pattern signal before the control switches from the normal speed pattern to the terminal slowdown speed pattern. Since there is about a 0.5 second delay in car response to the speed pattern, the stop under control of terminal slowdown is not always as accurate as desired.
Also, even under normal conditions, the actual speed of the elevator car may not exactly follow the desired speed pattern, and the tracking may be different from run to run. For example, as the elevator car drive machine heats up, or as the car load approaches maximum capacity, the elevator car speed may vary slightly with a consistent slowdown speed pattern. The car could make an acceptable slowdown and landing with this slight variation between speed pattern and actual car speed, but terminal slowdown may be activated. If the "speed error" between actual and desired speed required to initiate terminal slowdown is increased, to prevent nuisance or unnecessary activation of terminal slowdown, then the delay in switching to terminal slowdown, when it is actually necessary, becomes even longer. Finally, the notched blade is time consuming and therefore costly to manufacture because of the non-uniformly spaced teeth.